David Cameron’s gay marriage plans were plunged into crisis tonight by claims that one of his closest Cabinet supporters privately admitted that teachers and the Church of England could be sued.

The local Conservative chairman in Education Secretary Michael Gove’s Surrey Heath constituency said the minister had told him of his fears. “I’ve discussed this matter with Michael Gove on a number of occasions and we are concerned that this legislation has consequences,” Geoffrey Vero said.

“I think it has consequences for teachers, I think it has consequences for parents and children, and although Michael says in the Mail today that he has total confidence in the legislation, well that’s not what he told me only a week ago when I met him in Parliament.”

On the position of the Church, Mr Vero said: “Although they talk about the quadruple lock, we don’t have total confidence that that is going to stand the test of time.”

Mr Gove’s office responded by saying he did not believe teachers could be sued — but there was no denial of Mr Vero’s account.

Mr Vero’s claim was made to the John Pienaar show on Radio 5Live.

Critics of gay marriage said it cast real doubt over the effectiveness of the “quadruple lock” of laws meant to protect churches from being compelled to offer gay ceremonies. “I am not convinced that the consequences have been thought through,” said Enfield Southgate MP David Burrowes, who is campaigning against the Bill.

A massive rebellion is expected tomorrow in the first big Commons vote on the equal marriages Bill. In key developments today:

A group of London MPs wrote to colleagues and to the Evening Standard backing the Bill. Gavin Barwell, Jane Ellison, Mark Field, Mike Freer, and Richard Ottaway urged: “As Conservatives, we believe marriage is a force for good in society and we want as many as possible to benefit from it, while protecting religious freedom.”

Two Cabinet ministers are set to vote against: Environment Secretary Owen Paterson and Welsh Secretary David Jones. Welfare Secretary Iain Duncan Smith declined to say how he would vote and Defence Secretary Philip Hammond is expected to abstain.

Labour leader Ed Miliband gave the pro-equality campaign a push with a YouTube video in which he said: “I’ll be voting for equal marriage in the House of Commons and I’ll be doing so proudly.”

The PM was keeping a low profile. No 10 sources said reports that he was ringing Tory MPs to exhort them to support the Bill were incorrect.

Mr Gove’s spokesman did not comment on Mr Vero’s claims about the quadruple lock. But an education department spokesman said teachers will have “nothing to fear” from telling pupils they disagree with equal marriage.